The present invention concerns a thin film transistor.
Generally, a thin film transistor is used as a switching element for a device such as a plate display, since it realizes low voltage drive, low power consumption, light weight and high quality imaging. Such a thin film transistor will be briefly described with reference to FIGS. 1 and 2.
FIGS. 1 and 2 illustrate respectively a cross section and plan view of the construction of a thin film transistor of the prior art. First a metal thin film of Cr is deposited on a glass substrate A to form gate B, on which are successively deposited gate insulating layer C of an a-Si:N film, semiconductor layer D of an a-Si:H film, and ohmic layer E of n+ a-Si:H film by plasma assisted chemical vapour deposition (PACVD).
Thereafter, the semiconductor layer D of a-Si:H film and the ohmic layer E of n+ a-Si:H film are subjected to photolithographic process to form a pattern, on which are deposited source F and drain G. The source and drain are made of a second metal thin film of Al. Finally, the ohmic layer E of n+ a-Si:H film remaining in the channel between the source and drain is removed by dry etching, thus completing a thin film transistor.
Meanwhile, the etching speed of the gate insulating layer C of a-SiN:H is equal to that of the semiconductor layer D of a-Si:H, so that the gate insulating layer C of a-SiN:H may be etched simultaneously with the ohmic layer E of n+ a-Si:H and semiconductor layer D of a-Si:H being etched. Hence, the gate insulating layer C is exceedingly etched so as to lessen the insulating effect, subsequently causing a short circuit at the crossing of the gate and source leads. Moreover, there are generated pin holes when depositing the semiconductor layer D on the gate insulating layer C, so that the electrodes are short-circuited with each other when using the thin film transistor thus obtained as a switching element. Besides, when the thin film transistor is arrayed, as shown in FIG. 2, to make a display, the gate and source are crossed, thereby causing malfunction.